A polyester film is used in a magnetic recording tape because it has excellent thermal and mechanical properties. Magnetic recording tapes, especially data storage magnetic recording tapes require a larger storage capacity and a higher recording density as the size of data to be recorded is growing, and the requirements for the characteristic properties of base films constituting the magnetic recording tapes are becoming more severe.
Out of the magnetic recording tapes, magnetic recording tapes for data storage such as QIC, DLT, large-capacity super-DLT and LTO employ linear recording system (also called “linear track system”), and the track pitch is becoming very narrow to realize a large capacity and a high density for the magnetic recording tapes. The dimensional changes rate in the width direction of the magnetic recording tape changes, track dislocation occurs even if the dimensional changes rate in the width direction is so small that it does not cause a problem in the prior art, whereby an error occurs.
The dimensional changes rate in the width direction of the magnetic recording tape are caused by temperature and humidity variations and tension variations. To solve these the dimensional changes rate in the width direction dimensional changes rate in the width direction, the pamphlet of WO99/29488 proposes a biaxially oriented polyester film whose temperature expansion coefficient αt in the width direction (×10−6/° C.), humidity expansion coefficient αh in the width direction (×10−6/% RH) and shrinkage factor P (ppm/g) in the width direction for a load which is applied in the longitudinal direction are set to specific ranges. The pamphlet of WO00/76749 and the pamphlet of WO02/45959 also propose a biaxially oriented polyester film whose dimensional changes rate in the width direction when it is left under load in the longitudinal direction, temperature expansion coefficient αt in the width direction (×10−6/° C.), humidity expansion coefficient αh in the width direction (x 10−6% RH) and dimensional changes rate in the width direction (%) for a load which is applied in the longitudinal direction are set to specific ranges.
However, the requirements for large storage capacity and high recording density from magnetic recording tapes are becoming more severe nowadays and the biaxially oriented polyester films proposed by these patent documents cannot meet the requirements fully.